Fluid Catalytic Cracking (FCC) is one of the most important secondary conversion processes to convert the high-boiling, high-molecular weight hydrocarbon fractions to valuable products such as LPG, gasoline, and middle distillates. This process provides flexibility to maximize the product of interest through operation under different modes.
The conversion section of FCC unit consists of riser, reactor, stripper, regenerator and hardware internals. As the price of petroleum crudes has been increasing, more and more emphasis has been given on increasing the efficiency of the catalytic cracking process so that the end product can be more competitively priced. As a result, in the recent past, down flow reactor has been introduced to improve the product selectivity. U.S. Pat. No. 4,385,985 discloses downflow reactor for FCC process claiming it to be better compared to the upflow riser with respect to coke make, distribution of catalyst and decrease in contact time.
In down flow reactor, the catalyst along with the hydrocarbon vapors flow downwards and the slip velocity between the catalyst flow and the hydrocarbon flow is lower than that of the riser reactor resulting in reduced coke formation and higher distillate yield. Although the downflow reactor is said to have superior flow characteristics in comparison to the riser, the main challenge remains the distribution of the catalyst at the initial section of the down flow reactor where it is contacted with hot hydrocarbon feed. Moreover, the radial distribution of the catalyst in a downflow reactor deteriorates with increase in diameter.
Accordingly, Qi.et.al (1990) proposes a configuration where the gas solid flow after the riser is not separated but enters directly to the downer through a sharp 90° bend or smooth connection. Cao.et.al (1994) proposes a fluidized bed feeder designed to Pthe solids at the downer inlet. Solids are fluidized uniformly above the downer inlet and flow through several tubes into the downer.
U.S. Pat. No. 6,200,534 discloses a first circulating fluidized bed reactor surrounded by a second circulating fluidized bed reactor which is concentric to the first. The invention, however, is directed at reducing the footprint/space requirement of the equipment. Many chemical processes involve two separate catalytic or even physical processes. The design of the reactor combines two such processes within the same device, thereby achieving the goal of space saving.
U.S. Pat. No. 5,468,369 discloses a process and apparatus for short contact time fluidized catalytic cracking of heavy oil feed using a reactor with upflow cat:oil vaporizer and downflow reactors. It is claimed that efficient catalyst oil mixing with limited conversion is achieved in the upflow section and the cracking reactions are completed in the downflow section.
U.S. Pat. No. 5,296,131 discloses a concept of falling curtain of catalyst which is claimed to provide maximum exposure of catalyst to oil and difficult to control the flow. This patent describes about the annular curtain of downward flowing solids with flow controlled through a plug valve mechanism placed in the flow path of the catalyst. The option of feed injection with at least one part of the feed is injected toward the bottom through a radial opening under the seat of plug valve has been provided.
U.S. Pat. Nos. 4,814,067, 4,919,898, 5,098,672 and 6,482,312 B1 DIS disclosed by SWEC illustrates the formation of falling annular curtain from rectangular openings provided in a bed pressurized by steam. This patent describes the injection of feed in the direction of valve forming the curtain. This technique suffers from the solids flow control through the pressure difference. As for small pressure difference can cause in wide variation of flow which shall ultimately result in wide variation of selectivity.
From the above analysis of prior art, it is found that there is no disclosure of a compact integral hardware consisting of an annular curtain flow reactor to provide an efficient contact between circulating catalyst and hydrocarbon feed and efficient control of the same. The present invention discloses a process and apparatus for improved contact of catalyst and hydrocarbon feed leading to improved conversion and product selectivity. The apparatus in the current invention mainly comprises of radial distributor for catalyst and an annular downer reactor.